The present invention relates to an overdischarge and overvoltage protection circuit for a battery in a portable, mobile radio communication system, more particularly to an overdischarge and overvoltage protection circuit which rings alarming sounds and disconnects power supply for battery (preferably Lead-Acid battery) protection when the battery overdischarges, or automatically disconnects a power supply when an overvoltage is supplied while using an exterior power supply.
In general, Ni-Cd and Lead-Acid batteries are used mostly for a main power supply of a mobile communication system. By the way, the Lead-Acid battery is more employed than the Ni-Cd battery since the Lead-Acid battery is cheaper than the Ni-Cd battery.
FIG. 1 is a diagram of a conventional circuit, which consists of voltage comparators (Al) and (A2) of a battery protection (2) which detects a voltage-state and generates outputs to a control unit(3) when a voltage of a Lead-Acid battery is applied, and the control unit(3) which alarms and displays a warning signal through a speaker (6) and a display unit (6) respectively according to the detected voltage-state.
FIG. 2 is a diagram showing discharging characteristics of the battery, and an operational aspect will now be described with reference to FIGS. 1 and 2. A voltage of the Lead-Acid battery (1) is applied to non-inverting input terminals (+) of the comparators (A1) and (A2). At this time, if the voltage of the Lead-Acid battery is lower than a reference voltage, a signal for a low-battery (hereinafter referred to as Low-Bat), which is a logic low state of voltage of the battery, is generated by the voltage comparator (A1) and applied to the control unit (3) in order to generate a warning signal for recharge even though it is possible to use as the present voltage. If a given voltage of the Lead-Acid battery which is applied to non-inverting input terminals (+) is lower than warning voltage, the voltage comparator (A2) generates a warning signal for a battery termination (hereinafter referred to a Bat-Term), that is a logic low state which indicates the voltage of the Lead-Acid battery (1) reached out of an usable limit, thus cutting off the power supplying is necessary. This Bat-Term signal is applied to the control unit (3). When the Low-Bat signal from the voltage comparator (A1) is applied to the control unit (3), the control unit (3) generates alarming sounds and a warning sign which indicates a recharging is necessary since the voltage is low even though it is still usable through the speaker (6) and the display (5). However, if the Bat-Term signal from the voltage comparator (A2) is applied to the control unit (3), the control unit (3) generates alarming sounds and a warning sign, which indicates that the voltage of the Lead-Acid battery (1) has reached out of the usable limit and the main power supply has to be turned off, through the speaker (6) and the display unit (5) respectively. Also, in the mobile communication system, amounts of power consumption from the Lead-Acid battery (1) at a stand-by mode and a call mode are different. Especially, at the call mode, a transmitter "Tx" which sends out a radio carrier operates, thus more power is consumed than at the stand-by mode. Lead-Acid battery (1) must be able to supply different amounts of current for a warning level for Low-Bat and Bat-Term at the stand-by mode and for a warning level for Low-Bat and Bat-Term at the call mode according to the discharging characteristics diagram as in FIG. 2. To perform the above-mentioned operation, the control unit (3) shorts resistors "R3" and "R6" by switching on switches "SW1" and "SW2" at the stand-by mode to increase detection voltages of the voltage comparators (A1) and (A2). On the other hand, at the call mode, by switching off the switches "SW1" and "SW2", the detection voltages of the voltage comparators (A1) and (A2) become different.
The conventional circuit as shown in FIG. 1 has a drawback that there is no protection from overdischarge of the battery when a malfunction occurs due to a shock by an exterior power supply or a static electricity since it is not possible to operate a switch to disconnect the main power supply in the mobile communication system causing a short lifetime of a battery. The other drawback is that there is no protection from overvoltage when using an exterior power supply.